In an electron microscope, the specimen to be investigated is scanned in a meander-like manner with an electron beam focused on the specimen. This leads to various interactions on the specimen. Cathodoluminescence is understood to be the emission of quantums of light in the visible or neighboring spectral ranges by the electron beam. Images are generated, for example, on a monitor, with the aid of suitable detectors and an image buildup corresponding to the scanning movement of the electron beam. These images correspond to the light emission of the specimen. In this connection, specific spectral regions can be separated out, for example, by means of a filter. Often, also other interactions between the electron beam and the specimen are simultaneously investigated with additional detectors, for example, secondary electrons.
East German Patent 207,994 discloses a cathodoluminescence detector wherein a paraboloid mirror is mounted between the pole shoe of the last electron optical lens and the specimen. The parabolic mirror has a bore for the electron beam and reflects the light emanating from the specimen lying in its focal point as a parallel light handle onto a lens which concentrates the light bundle onto a photoelectric receiver. A disadvantage of this arrangement is that the paraboloid has to be relatively large for a good exploitation of the light and therefore the specimen must be inclined toward the electron beam. As a consequence of the foregoing, the dimensions of the specimen are limited.
Reference may also be made to an article entitled "Verbessertes Ellipsenspiegel-Detektorsystem fur die Kathodolumineszenz-Rasterelektronenmikroskopie" by E. M. Horl appearing in the journal entitled "Beitrage elektronenmikroskopischer Direktabbildungen von Oberflachen 8, page 369 (1975). This publication discloses a cathodoluminescence detector wherein the specimen is mounted in the one focal point of an elliptical mirror and wherein a parabolic mirror is additionally mounted for the best possible light exploitation such that its focal point lies in the second focal point of the ellipse. Although the specimen here must not be inclined with respect to the electron beam, its magnitude is, however, likewise limited by the parabolic mirror. This publication also discloses the use of a light conductor as a vacuum window.